Nether Portal spawning

Built first nether protal and no problem transporting between environments. Built 2nd portal some distance away, transported fine but decided to destroy that one. Moved on and built another (2nd) portal, transported ok but when I came back it was through a portal that had spawned itself in a cave system not too far from the location of the portal I destroyed. Anyway, destroyed the spawned portal by removing a couple of bricks, went back to new portal and tried again. Same thing happened, another portal had spawned right next to the one I had destroyed. Is this a bug or a part of the game play? Very confusing and am disinclined to built further portals.